Print this pageAbstract: Principles
of 
AVO
Crossplotting
CASTAGNA*, JOHN P. and HERBERT W. SWAN,
Hydrocarbon related
"AVO anomalies" may show increasing or decreasing amplitude variation with
offset. Conversely, brine-saturated "background" rocks may show increasing
or decreasing AVO. Amplitude-versus-offset interpretation is facilitated
by crossplotting AVO intercept (A) and gradient (B). Under a variety of
reasonable geological circumstances, A's and B's for brine-saturated sandstones
and shales follow a well-defined "background" trend. "AVO anomalies" are
properly viewed as deviations from this background and may be related to
hydrocarbons or lithologic factors. The common three-category classification
of gas-sands developed by Rutherford and Williams is incomplete. We propose
that an additional category (Class IV) be considered. These are low impedance
gas-sands for which reflection coefficients decrease with increasing offset;
they may occur, for example, when the shear-wave velocity in the gas sand
is lower than in the overlying shale. Thus, many "classical" bright spots
exhibit decreasing AVO. If interpreted incorrectly, AVO analysis will often
yield "false negatives" for Class IV sands. Clearly, the conventional association
of the term "AVO anomaly" with an amplitude increase with offset is inappropriate
in many instances and has led to much abuse of the AVO method in practice.
Similarly, interpretation of partial stacks is not as simple as looking
for relatively strong amplitudes at far offsets. We recommend that all
AVO analysis
be done in the context of looking for deviations from an expected
background response.
AAPG Search and Discovery Article #90944©1997 AAPG Mid-Continent Section Meeting, Oklahoma City, Oklahoma